MAYAH Merk II, Руководство пользователя

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Prepare Connection/Transfer
54
© MAYAH Communication GmbH
6.2. Prepare Connection/Transfer
6.2.1. Mix Input Signals and Adjust Level
Before you set up the connection to your communication partner, we recommend that
you set the signals to be transmitted suitably.
6.2.2. Select the correct bit rate
Merk II supports the procedures G.711, G.722, Layer 2, Layer 3, AAC, MPEG 4 AAC Low
Delay and ADPCM 4SB (see table below), which behave very differently as far as quality
and the required bit rate is concerned. Particularly in the case of coding standards based
on a psycho-acoustic procedure such as L2, L3 and AAC, there is no longer the direct
relationship between audio bandwidth = sampling rate / 2. Rather more, when using MPEG
standards it is necessary to find the best compromise for the audio bandwidth for each
combination of a bit rate with an sampling rate and a mode. This particular applies with
respect to the very low data rates, since the maximum bandwidth can always be used for
high data rates.
Furthermore, the sampling rate with which the encoder algorithm works does not automati-
cally correspond to the sampling rate with which the digital AES/EBU input has to work. In
order to adapt the MERK II to e.g. a 96 kHz studio environment in this case, the devices
have sampling rate converters. It is thus possible to set the sampling rate of the encoder to
the optimum value for the transmission procedure and bit rate independent of the
environment.
How the sampling rate and the bandwidth of a monophonic signal at a bit rate of 64 kBit/s
relate to each other, for example in the case of Layer 2, is shown in the following table:
Sampling rate (kHz) 16 24 32 48
Bandwidth (kHz) with Layer 2
7.5 10.25 9.0 8.25
You can see that the best sound quality with Layer 2 is achieved with a sampling rate of 24
kHz. At a higher sampling rate, the available bandwidth is reduced even more. If a
specification states a bandwidth of 20 kHz at a data rate of 64 kBit/ s and a sampling rate
of 48 kHz, the best possible sound quality is not to be expected.
If you want to find out more about these relationships, we recommend you study the
appropriate literature on the subject.
The following sampling rates are recommended as a matter of principle:
Mono Stereo
64 kBit/s 128 kBit/s 64 kBit/s 128 kBit/s 192 kBit/s 256 kBit/s
G.711
8 kHz - - - - -
G.722
16 kHz - - - - -
Layer 2
24 kHz 32 kHz 16 kHz 32 kHz 48 kHz 48 kHz
Layer 3
32 kHz 32 kHz 24 kHz 32 kHz 48 kHz 48 kHz
AAC
48 kHz 48 kHz 48 kHz 48 kHz 48 kHz 48 kHz
MPEG 4 AAC
Low Delay
*)
48 kHz 48 kHz 48 kHz 48 kHz 48 kHz 48 kHz
ADPCM 4SB
*)
- 32 kHz - - - 32 kHz
Stand. APT-X
16 kHz 32 kHz 8 kHz 16 kHz 24 kHz 32 kHz
Enh. APT-X 16
16 kHz 32 kHz 8 kHz 16 kHz 24 kHz 32 kHz
Enh. APT-X 20
12,8 kHz 25,6 kHz 12,8 kHz 19,2 kHz 25,6 kHz
Enh. APT-X 24
10,67 kHz 21,3 kHz 10,67 kHz 16 kHz 21,3 kHz
*)
if released